1. Field of the Invention
The present invention relates to a thin-film photo-sensing element formed on an insulating film substrate and to a photo-sensing device using the same. In particular, the invention relates to an optical sensor array such as an X-ray imaging device, a near-infrared light detector for biometrics, etc. and an image display unit integrated with a display panel with touch panel function, ambient light detecting function, and input function using photo-sensor, e.g. low temperature process semiconductor thin-film transistor used in liquid crystal display, organic electroluminescence display, inorganic electroluminescence display, and electro chromic display, and low temperature process photoconductive element or low temperature process photo-diode element.
2. Description of the Prior Art
X-ray imaging device is now indispensable as a medical treatment device, and there are strong and continuous demands to simplify the operation of the device and to produce it at lower cost. Also, in recent years, special notice has been given on the means for biometrics to obtain information from finger vein or palm vein, and it is an imminent problem to have a device or a system for reading the information of this type. In such device or system, a sensor array for optical detection in a certain area, i.e. the so-called area sensor, for reading these types of information, and this area sensor must be provided at lower cost. To satisfy these requirements, a method has been proposed in the Non-Patent Document 1 as given below, according to which an area sensor is prepared by semiconductor forming process (planer process) on an inexpensive insulating substrate typically represented by glass substrate.
In the field of the products different from the area sensor, the photo-sensor is also required on a medium-to-small size display. The medium-to-small size display is applied for display purpose in mobile devices such as handy phone, digital still camera, PDA (personal digital assistant), or display on board. Multiple functions and high performance characteristics are required on these types of devices and systems. Attention is now focused on the photo-sensor as effective means for adding ambient light detecting characteristics (see the Non-Patent Document 2 given below) and touch panel functions. However, unlike the large size display, panel cost is low in the medium-to-small size display. This means that the cost is increased for mounting the photo-sensor or the sensor driver. Therefore, when a pixel circuit is prepared on a glass substrate by semiconductor forming process (planer process), special notice is now given on the technique to prepare the photo-sensor or the sensor driver and on the method to manufacture them at lower cost.
The important issues in the groups of products as described above are to prepare a photo-sensing element or a sensor driver on an inexpensive insulating substrate. The sensor driver typically comprises LSI, and it usually requires MOS transistor deposited on monocrystalline silicon wafer or a switching element with high performance characteristics of similar type. To solve such problems, the technique as described below seems to be essential.
As pixel and pixel driver circuit element for an active matrix type liquid crystal display, an organic electroluminescence display, or an image sensor, the thin-film transistor (hereinafter referred as “polycrystalline semiconductor TFT”) has been developed, which is made up by polycrystalline semiconductor. Compared with other types of driver circuit elements, the polycrystalline semiconductor TFT is advantageous in that it has higher driving ability. Peripheral driver circuit can be prepared on the same glass substrate as pixel. As a result, this is convenient for attaining the customization of circuit specification and low cost production by simultaneously performing pixel designing and preparation process and for achieving high reliability by avoiding mechanical fragility of the connections of the driving LSIs and pixels.
The polycrystalline semiconductor TFT for liquid crystal display is prepared on a glass substrate for the purpose of reducing the manufacturing cost. In the process to prepare TFT on the glass substrate, process temperature is determined by heat-resistant temperature of the glass. As a method to prepare polycrystalline semiconductor thin-film of high quality without giving thermal damage to the glass substrate, ELA method (excimer laser annealing method) is known, according to which the semiconductor layer is molten and re-crystallized. The polycrystalline semiconductor TFT obtained by this method has driving ability more than 100 times as high as that of TFT (with the channel made of amorphous semiconductor) as used in the conventional type liquid crystal display, and some of the circuits such as driver circuit can be mounted on the glass substrate.
With regard to the photo-sensor, a method to use the polycrystalline semiconductor TFT and a method to use a PIN type diode in addition to pixel circuit and driver circuit are described in the Patent Document 1 as given below. The characteristics required for the photo-sensor are high sensitivity and low noise. If it is limited to the important characteristics of the photo-sensing element, “high sensitivity” means to issue as high signal as possible with respect to a light with certain intensity. This means that a material and an element structure with high light-to-current conversion efficiency are required. “Low noise” means that the signal is as low as possible when the light is not projected.
FIG. 12 represents cross-sectional views each showing a conventional type photo-sensing element. FIG. 12 (a) shows a PIN type diode element of vertical structure using amorphous silicon layer as a photoelectric conversion layer 113. FIG. 12 (b) shows a TFT element of lateral structure, which uses amorphous silicon film as the photoelectric conversion layer 113 and in which electric charge flows in parallel to the connected surface. Both of these serve as photo-sensing elements.
The photo-sensing element as shown in FIG. 12 (a) comprises a photoelectric conversion layer 113 of amorphous silicon film interposed between a first metal electrode layer 111 and a second metal electrode layer 112, and an impurity induced layer 120, which is prepared between the photoelectric conversion layer 113 and each of the electrode layers. This photo-sensing element is disposed on an insulating substrate 110. Each of the electrode layers is connected to an electrode line 114 insulated by an interlayer insulating film 115 and is covered with a passivation film 117.
The photo-sensing element shown in FIG. 12 (b) comprises a source electrode 131, a gate electrode 132, a drain electrode 133, a gate insulating film 134 and a photoelectric conversion layer 113 made of amorphous silicon film. Further, it comprises an impurity induced layer 120 disposed on boundary surface between the photoelectric conversion layer 113 and each of the electrodes. This photo-sensing element is mounted on the insulating substrate 110 and is covered with a passivation film 117.
In FIG. 12, as a semiconductor material to be used in the photoelectric conversion layer 113 of the sensor element disposed on the insulating substrate 110, a silicon type material such as silicon, silicon-germanium, etc. should be used because due consideration must be given on environmental problem or process coordination when driver circuit (or pixel circuit) is formed at the same time. When the silicon type material is used, among the light components absorbed in the wavelength range from infrared light to visible light, almost all of the light components are converted to electric current. This means that a material having higher absorption coefficient is suitable as the material for the photo-sensing element.
Also, when attention is given on solid phase status (hereinafter referred as “phase status”) of semiconductor such as amorphous, crystalline or polycrystalline semiconductor, absorption coefficient of the amorphous material is at the highest for the entire wavelength range and this has high resistance. In this respect, amorphous material is advantageous and suitable as the material of the sensor element.
However, when the amorphous material is used in the sensor element, the performance characteristics of the switching element are not sufficient, and it is not possible to have the driver circuit at the same time. For instance, when TFT is made of amorphous silicon material, which is optimal as the material for the sensor element, field effect mobility is 1 cm2/Vs or lower. For this reason, high sensor characteristics can be attained by preparing the sensor array as the structure shown in FIG. 12, while switching characteristics can be provided by mounting the driver LSI and by connecting via the means such as FPC.
When the material is monocrystalline, it is possible to make up the sensor element and the circuit at the same time. This manufacturing process is a process requiring the temperature as high as 1000° C. or higher. Thus, it cannot be prepared on an inexpensive insulating substrate such as glass substrate.
When the switching element and the sensor element to constitute the driver circuit are made of polycrystalline semiconductor film, the driver circuit (and also, pixel circuit) and the sensor element can be prepared at the same time on the same insulating substrate. In case of the polycrystalline semiconductor film produced by ELA method, TFT with high quality can be obtained, which can be used for the driver circuit.    [Non-Patent Document 1] “Technology and Applications of Amorphous Silicon; pp. 204-221.    [Non-Patent Document 2] SHARP Technical Journal, Vol. 92 (2005); pp. 35-39.    [Patent Document 1] JP-A-2006-3857.